1. Technical Field
The present invention relates to an image forming apparatus and an image forming method for use in electrophotographic image formation. More specifically, the present invention relates to an image forming apparatus and an image forming method for use in electrophotographic image formation, which are used in a field of copying machines or printers.
2. Description of Related Art
An organic photoreceptor is greatly advantageous in that the selection range of materials is wide, the environmental suitability is excellent, the production cost is low, and the like, as compared with an inorganic photoreceptor such as a selenium photoreceptor or an amorphous silicon photoreceptor. Recently, the main stream of the electrophotographic photoreceptors is shifted to organic photoreceptors in place of inorganic photoreceptors.
On the other hand, in image forming methods according to Carlson method, the electrophotographic photoreceptor is charged to form thereon a static latent image, the static latent image is visualized to form a toner image, and the toner image is transferred to transfer paper and fixed, whereby a final image is formed.
A corona discharger is best known for a charging member conventionally used typically as a member of the charging means. The corona discharger has an advantage in that stable charging can be carried out. However, the corona discharger has problems in that high voltage must be applied and therefore, ionized oxygen, ozone, moisture, nitrogen oxide compounds and the like are largely generated, as a result, deterioration of an organic photoreceptor (hereinafter, also refers to as a photoreceptor) is incurred or adverse effects are caused on human bodies.
Consequently, employment of a contact charging method using no corona discharger is recently considered. More specifically, the contact charging method is a system in which a magnetic brush or an electroconductive roller as a charging member is brought into contact with a photoreceptor as a body to be charged while applying a voltage to the brush or the roller and thereby a surface of the photoreceptor is charged to have a predetermined potential. When using such a contact charging method, an applied voltage is reduced and an ozone generation amount is decreased, as compared with a non-contact charging method using a corona discharger.
The contact charging method is a method in which a charging member having a resistance of about from 102 to 1010 Ω·cm is brought into contact with a photoreceptor under pressure while impressing a DC voltage formed by superposing DC or AC to the member and thereby charges are imparted to the photoreceptor. The charging method is carried out by discharge from a charging member to a body to be charged, in accordance with a Paschen's law and therefore, the charging is started by impressing a voltage of certain threshold or more. When using this contact charging method, an impressed voltage to the charging member is reduced and a generation amount of ozone and nitrogen oxides is decreased, as compared with the corona charging method.
However, when the charging is repeatedly performed on the surface of an electrophotographic photoreceptor by a direct contact with the charged roller, etc., tiny asperities or contaminants occur on the electrophotographic photoreceptor. As a result, charges concentrate at the asperities or contaminants, which readily causes formation of image defects such as dielectric breakdown or black spots and occurrence of image blurring. These problems readily occur, in particular, under strict conditions of high temperature and high humidity, or low temperature and low humidity.
Further, a cleaner-less image forming apparatus using no cleaning blade for removing residual toners on the photoreceptor with the contact charging is disclosed (JP-Tokukai-2000-199990A). The image forming apparatus has auxiliary charging section in addition to the charging section. The auxiliary section has a function of charging residual toners and enhancing recovery efficiency of the residual toners by developing section. On the contrary, when the auxiliary charging section is provided, the above-described dielectric breakdown or black spot is more readily caused.
It is proposed that in order to prevent formation of the above-described image defects such as dielectric breakdown or black spot, an aluminum substrate surface of a conductive support is subjected to alumite treatment to enhance a resistance force to charge leakage of the electrophotographic photoreceptor, and even if asperities or contaminants occur on the photosensitive layer, the charge leakage from the conductive support is prevented (JP-Tokukaihei-5-80567A).
However, the electrophotographic photoreceptor using the alumite treated aluminum substrate has a problem that an alumite layer is deteriorated due to the alumite treatment and a subsequent slight change of aging conditions, and as a result, an effect of preventing the above-described charge leakage is hardly obtained stably. In addition thereto, the photoreceptor has a problem that a portion between an alumite layer and a photosensitive layer is readily made to function as a charge trap site and residual potential tends to be gradually accumulated due to a long-term use.
An object of one aspect of the invention can be to provided an image forming apparatus and an image forming method, where a charging method of reducing the generation amount of ozone or nitrogen oxides and consuming small electric power is used and stable image formation can be performed for a long time.
Another object of one aspect of the invention can be to provide an image forming apparatus and an image forming method, in which an organic photoreceptor is used for an electrophotographic photoreceptor for use in the image forming apparatus employing a contact charging method, deterioration in electrophotographic characteristics (e.g., sensitivity or residual potential), which is readily generated while the organic photoreceptor is repeatedly used, is prevented, formation of image defects such as dielectric breakdown or black spots is prevented, and stable image formation exhibiting excellent sharpness can be performed for a long time.